digikam from trunk:
Synchronize display of tags, date, comment and rating
in comments&tags sidebars, main view, and tag filter and folder views

- add class ImageAttributesWatch to multicast information
about changes of these attributes
- connect all relevant parts to the signals of this class
- add calls to this class at all places where these
attributes are changed (exception: caption and date in
kipiinterface, where no imageId is available)